1. Field of the Invention
The instant invention relates to gibs for low friction, linear motion of one structure of a machine with respect to another. More specifically, the invention relates to lightweighting of the linear guideway portion of the gib for use in high acceleration applications such as in semiconductor fabrication.
2. Description of the Related Art
Bearings are used in machines to provide low friction, uniform motion of one or more components relative to one or more other components. Bearings can be categorized as “contact” or “non-contact”. The latter category includes the gas bearings and those bearings that rely upon the maintenance of a hydrodynamic film for their operation. Contact bearings, on the other hand, feature an actual physical contact between the components that move with respect to one another. Slide bearings specifically provide for low friction linear motion, and are sometimes referred to as “linear bearings”.
Gibs are machine elements associated with slide bearings of the contact type that help to ensure a high degree of precision and control in this linear motion. Such precise control of movement or position is required in operations such as machining or semiconductor fabrication. In the literature, the term, “gib” sometimes refers only to a tapered beam that can be pressed against a slide or linear bearing assembly to take the slack or “play” out of the latter, which can arise due to wear in the bearings, guides or ways. In the context of the instant invention, however, “gib” may still include this function, but primarily refers to the entire collection of parts that is responsible for the low-friction, high-precision straight-line motion of one piece of a machine relative to another piece. Typically, a plurality of gibs is required in each machine, as motion usually must be controlled in more than one dimension or degree of freedom. In addition, machines often require more than one gib per travel dimension.
Gibs come in a number of varieties. Some forms feature low friction polymers such as polytetrafluoroethylene (PTFE) at the contact surface. Other versions have ball bearings or roller bearings sandwiched between two (or more) hard, flat surfaces. The surfaces often function as housings or tracks for the bearings, with one of the housing halves attached to, for example, the base of the machine, and the other housing half attached to the table, stage or surface that is to be in motion relative to the base. The hard bearing housings (hereinafter referred to as “guides” or “guideways”) are typically made from tool steel.
In certain applications, such as CNC machining or semiconductor wirebonding, the faster the workpiece can be moved, the more productive is the machine. Moving the workpiece about implies that it is to accelerated/decelerated, and the magnitude of such accelerations is governed by F=m a, “force equals mass times acceleration.” The applied force correlates with the power of the motor driving the movable stage on which the workpiece is mounted. For a driving motor of given power, the speed to which the workpiece can be accelerated is inversely proportional to the mass to be accelerated. Thus, greater speeds and therefore greater manufacturing efficiencies can be realized by reducing the mass to be moved. Considerable progress has been made in reducing the mass of the stage or table in machines that fabricate and package integrated circuits or “IC's”. Lightweight materials such as aluminum or composites have replaced traditional but heavy steels and cast irons where possible. Accordingly, in an IC wirebonding machine, for example, the stage supporting the semiconductor “chip” to be wire bonded may have a mass of only a few kilograms. Until now, however, not much effort has been expended in reducing the mass of the gibs that make possible the low friction x-y motion of the stage. Thus, the gibs may make up a significant fraction of the total mass of the stage.
U.S. Pat. No. 3,953,086 to Chaffin discloses a tapered gib for machine tools that is composed entirely of a plastic material such as an acetal resin containing uniformly dispersed, finely divided particles of polytetrafluoroethylene to provide reduced friction. The surface of the gib that is in contact with the table features raised portions such as nibs, ridges, etc. The gib may be operated without lubrication and wears in preference to the metal components with which it is in sliding contact. Thus, elimination of the wear-induced “play” involved replacement of the gib, rather than replacement of the metal components contacting the gib, such as the table. This gib, however, possesses frictional resistance greater than that of a roller or ball type bearing, and the inventors highlight the fact that the friction is sufficiently high to be able to locate the table of a machine tool with a static friction lock. It would seem that in an application where the table must move at high speeds, such as in a wirebonding application, a gib having less friction would be preferable.
U.S. Pat. No. 5,096,348 to Winkler et al. discloses a machine tool guide comprising plastic sliding pads placed at the sliding surface of the tool guide. This transition from metal/metal contact to metal/plastic contact has a natural self-damping effect with respect to any vibrations that may be generated such as, for example, by a machining operation. Channels are provided in the plastic pads for introduction of a lubricant such as oil so that the frictional forces that are encountered may be minimized.
It seems that many of the plastic based bearings require additional lubricants to further reduce their coefficients of friction.
U.S. Pat. No. 4,877,813 to Jinno et al. discloses a composition for a so-called “plastic bearing” that is intended to function as a lightweight form of a metal bearing. The improvement comprising the addition of short, unentangled fibers of an aromatic polyamide to a thermoplastic resin (i) to improve the load capacity of the bearing compared to non-fiber-containing compositions, and (ii) to improve (lower) the friction coefficient compared to compositions employing conventional fibers such as glass or carbon. Although the inventors claimed that the bearing could operate dry, the coefficient of friction still was appreciable in this condition (about 0.34 to 0.36), but was improved considerably with additions of lubricating oil up to about 10 percent by weight.
In some applications, the requirement for oil lubrication poses a potential contamination issue. For example, U.S. Pat. No. 5,909,705 to Short et al. discloses a novel gib mount assembly comprising a squeeze seal oil film between the bearing surface of the gib mount and the adjoining slide used to control the clearance between the same. One or more vacuum regions are created about the boundaries of the oil film to suction and capture any oil escaping from the oil film region, thereby preventing contact of oil with the parts being processed by the machine (here, in a pressing or stamping operation).
Thus, efforts to produce lightweight gibs have focused on introducing polymeric materials or composites thereof into the sliding contact portions of the gib. In general, these gibs generate more friction than do their steel counterparts. For low friction applications, lubricants may have to be added, which is inconvenient and may pose contamination problems for workpieces that must be kept clean. Also, the polymers have much lower elastic moduli than steels, and accordingly undergo greater distortion than do steels for a given load. This property would seem to be at odds with current trends in semiconductor fabrication in which higher modulus materials such as metal or ceramic composites are being introduced to reduce the distortion under load and thereby achieve greater precision in motion control. Thus, it seems that the desired lightweight gibs should not rely on low modulus materials.
Many reinforced metals, or metal matrix composite (“MMC”) materials, possess a number of attractive attributes for a bearing/gib application, specifically low specific gravity compared to steels, and high elastic modulus. Many of these MMC's are reinforced with ceramic materials, and accordingly are quite wear resistant in dynamic contact with metals. One general problem with using MMC's as bearing materials is the lack of knowledge or experience in engineering or refining the material for this type of application. For example, many of these MMC's, particularly those employing silicon carbide as reinforcement, are simply too abrasive of the steel ball or roller bearing materials in which they are in contact.
Accordingly, other investigators have experimented with depositing various hard, high modulus coatings onto MMC surfaces in an effort to render the surface amenable to the high stress contact of a rolling-type bearing. The instant inventors are not aware of any such work that yielded a successful bearing-compatible surface. For example, while it is possible to plasma spray tool steel, such material plasma sprayed onto MMC turns out to be too porous to function properly as a guideway for a bearing in rolling contact.
The prior art steel guideways against which the hardened steel bearings run have excellent low friction and wear resistance properties. Further, a large experience base exists for steels, including those grades suitable for bearing applications. The problem with steel is its mass; specifically, tool steel may have almost three times the mass of a reinforced aluminum composite component of the same volume. What is needed is a gib that retains the beneficial properties of tool steel, such as low friction and wear resistance, but features the low specific gravity and high modulus of more advanced materials, such as composites.